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ASME B1.21M-1997 (2018) — Metric Screw Threads (MJ Profile)
🚀 ASME B1.21M-1997 (2018) covers metric screw threads of the MJ profile. The MJ thread is the hard-metric counterpart of the inch UNJ system (ASME B1.15 / MIL-S-8879): the external thread carries a controlled root radius of 0.15011P to 0.18042P, and the internal thread minor diameter is truncated to accommodate the maximum external root radius. The thread is recommended for high-temperature or high-fatigue applications and is the standard thread of aerospace metric fasteners. This edition is a revision of ANSI B1.21M-1978, approved as an American National Standard on December 9, 1997, issued April 2, 1998, and reaffirmed in 2018.
💡 One-line understanding of MJ: it transplants the UNJ ideas — a large controlled root radius plus a truncated internal minor diameter — into metric units, so metric aerospace designs get fatigue performance equivalent to the inch UNJ system.
1. Scope and Thread Form 📐
The Standard covers the basic profile, design profiles, root radius of the thread, series of threads, thread characteristics (length of thread engagement LE, tolerance system, limits of size, thread form tolerances), and dimensional accommodation of coatings and platings (external threads at tolerance positions h and g; internal threads at positions H and G). The key difference from the conventional metric M profile (ASME B1.13M) is that the external root radius is controlled between 0.15011P and 0.18042P, and the internal minor diameter is truncated to a basic depth of 9/16 H (75% h) to suit the radius. ISO Subcommittee for Aerospace Bolts and Nuts (ISO/TC 20/SC 4) has accepted the MJ profile for aerospace metric threaded fasteners.
| Tolerance Class | Application | Inch / Related Standard | Remarks |
|---|---|---|---|
| 4H5W4h6h | Aerospace applications (default) | Approx. equivalent to inch Class 3A3B J thread | External position 4h6h; internal 4H5W |
| 4g6g / 4G6G·4G5G | Commercial, or where the standard coating reduction is inadequate | Comparable to Class 2 UNJ of ASME B1.15 | 4G6G/4G5G for coated internal threads |
| 6g / 6H | Commercial applications only | Table 6 added in this issue defines limiting dimensions | For commercial producers and users |
| 6W6g | General Metric J applications | Comparable to the M profile of ASME B1.13M | Extends “Metric J” to general use |
⚠️ Class-selection red line: 4H5W4h6h is the aerospace-designated class; 6g/6H is for commercial use only and must not be used on aerospace load-bearing parts. When coatings change effective size, reserve positions H/G or g/h per Chapter 5 — never widen tolerances on your own.
2. History: From SAE AS 1370 to an American National Standard 📖
Because the UNJ inch thread predominated in aerospace design, the aerospace industry decided to produce a hard-metric version for metric aerospace design. The MJ thread project was initiated by SAE Committee E21 (aerospace propulsion design standards) at its October 25, 1971 meeting (Project E21-122). A metric thread panel was organized with representation across the industry: SAE E21/E25, AIA/NASC (airframe), ATA, SBAC, ALMA, APFA, ASD of the U.S. Air Force, and NAPTC of the U.S. Navy, with liaison to ANSI/OMFS and coordination with ISO/TC 20/SC 4 and ISO/TC 1/SC 5. After fourteen panel meetings that resolved metric diameter-pitch combinations, the thread design form, and tolerance classes and formulas, the result was balloted and approved as SAE Aerospace Standard AS 1370 (May 1976). Two preliminary documents laid the groundwork: AS 1337 (March 1974) listing aerospace diameter-pitch combinations for threaded fasteners, and AS 1338 (February 1976) documenting MJ profile requirements and tolerance classes; AS 1421 (August 1976) later added combinations for shaft and bearing retaining screw threads.
SAE submitted AS 1370 to ANSI Committee B1 for approval as the American National Standard for MJ metric screw threads. ANSI B1.21 was approved on February 23, 1977, and formally designated an American National Standard on November 21, 1978. This 1997 issue adds: tolerance classes 4G6G and 4G5G/4g6g (comparable to ASME B1.15, which includes Class 2 UNJ threads); tolerance class 6W6g, a Metric J profile comparable to the M profile of ASME B1.13M for general applications; and changes in rounding procedures that follow the principles of ASME B1.30M.
✅ The MJ thread is the de facto metric aerospace standard: from diameter-pitch combinations (AS 1337), profile and tolerance (AS 1338), and the parent standard (AS 1370 / B1.21M) to international adoption by ISO/TC 20/SC 4 — a complete traceability chain.
3. Dimensional Accommodation of Coatings and Platings 🔧
As in B1.15, the Standard specifies accommodation rules for coated external threads (tolerance positions h and g) and coated internal threads (positions H and G), plus dry-film lubricant coating cases. Key points:
- External threads at positions h / g: h is the aerospace-favored zero-basic-deviation position; g reserves space for coatings so the coated functional size returns within the class limits.
- Internal threads at positions H / G: H is zero-basic-deviation; G reserves space for coatings.
- Dry-film lubricants: include the film thickness in the tolerance allocation to avoid post-coating nonconformance.
⚠️ Metric plating has the same “won’t assemble after plating” or “loose after plating” traps: whether 4H5W4h6h or 6g/6H, first select the position letter (g, G) for the coating thickness, then apply the tolerance class digits.
4. Thread Designation and Engineering Practice 🛠️
MJ thread designations carry the MJ prefix in front of the M-form designation, e.g., MJ10×1.25-4h6h denotes an MJ external thread with 10 mm nominal diameter, 1.25 mm pitch, and tolerance class 4h6h; an internal thread would be written e.g. MJ10×1.25-4H5W. Chapter 6 gives the basic designation, standard series designation, and coated-thread designation rules.
- 🚀 Aerospace design: choose diameter-pitch combinations from the AS 1337/AS 1421 series; designate load-bearing fasteners as 4H5W4h6h and verify the root radius.
- 📏 Form control: the external root radius must lie within 0.15011P–0.18042P and the internal minor diameter must be truncated to 75% h; judge acceptance by functional size, not by a single pitch diameter.
- 🔩 Coating coordination: coating thickness and the position letter (g/G) must appear together on drawings and purchase documents, consistent with the B1.30M rounding rules.
- 📋 Gaging: gages and gaging practice for MJ product threads follow ASME B1.22M, Gages and Gaging for MJ Series Metric Screw Threads.
💡 Design lookup order: decide aerospace vs commercial first, then pick the class (4H5W4h6h or 4g6g/6g), then add the coating position letter — this avoids rework on tolerance digits.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between the MJ profile and the regular M profile (B1.13M)?
A: MJ controls the external root radius between 0.15011P and 0.18042P and truncates the internal minor diameter to a 75% h basic depth, giving lower stress concentration and longer fatigue life; the M profile has no such mandatory radius range. The 6W6g class of this Standard is a “Metric J” comparable to the M profile.
A: MJ controls the external root radius between 0.15011P and 0.18042P and truncates the internal minor diameter to a 75% h basic depth, giving lower stress concentration and longer fatigue life; the M profile has no such mandatory radius range. The 6W6g class of this Standard is a “Metric J” comparable to the M profile.
❓ Q: Which tolerance class should aerospace use?
A: 4H5W4h6h, approximately equivalent to the inch Class 3A3B J thread, is the class designated for aerospace. 6g/6H is commercial only; 4g6g/4G6G/4G5G serve commercial use or thicker coatings where the standard reduction is inadequate.
A: 4H5W4h6h, approximately equivalent to the inch Class 3A3B J thread, is the class designated for aerospace. 6g/6H is commercial only; 4g6g/4G6G/4G5G serve commercial use or thicker coatings where the standard reduction is inadequate.
❓ Q: Can MJ and UNJ be mixed?
A: They belong to metric and inch systems respectively with different diameter-pitch frameworks, so they cannot be interchanged directly; however, their design philosophy (root radius, truncated internal thread) and class systems are analogous, and cross-border programs should adopt one system consistently.
A: They belong to metric and inch systems respectively with different diameter-pitch frameworks, so they cannot be interchanged directly; however, their design philosophy (root radius, truncated internal thread) and class systems are analogous, and cross-border programs should adopt one system consistently.
🔍 Q: How are coated threads handled dimensionally?
A: Choose external position h (uncoated) or g (coating allowance) and internal position H or G; if the standard reduction is insufficient for the required coating thickness, move to coating-friendly classes such as 4G6G/4G5G, and always accept on the coated functional size.
A: Choose external position h (uncoated) or g (coating allowance) and internal position H or G; if the standard reduction is insufficient for the required coating thickness, move to coating-friendly classes such as 4G6G/4G5G, and always accept on the coated functional size.
